Ultra-thin wafers can reduce total carbon foot print of solar cells

The carbon foot print of crystalline solar cells has continuously decreased over the years due to advancement in technology in different stages of the value chain.  Recent studies have shown that life-cycle green house emission of PV systems based on crystalline silicon is as low as 37 g CO₂-eq/kWh¹.  This is only 4% compared to coal based power plants where green-house emission can be ~900 CO₂-eq/kWh^1,2.  With these figures, the energy payback of PV systems can be less than 3 years.  Still there are significant opportunities to reduce this further; one among them is the use of ultra-thin high efficiency wafers which can have an immediate effect. 

The most energy consuming step in the PV value chain is the polycrystalline manufacturing process followed by crystal growing step which can combinedly consume up to 150 kWh/kg.  Therefore, reducing the silicon uses in the module will have the greatest impact in reducing the carbon foot print of the PV systems.  Currently, the wafers used in crystalline solar cells are 180-200 micron thick and use about 6-8 grams of silicon.  Reducing the wafer thickness combined with better yield control and conversion efficiency can bring this down to ~2gm/watt.    This will significantly decrease the electricity use, hence the CO₂ emission for wafer production (Fig 1).  Since, there is no other CO2 emission during electricity generation from the PV system, this will significantly reduce the life time green house gas emission of PV electricity (Fig 2).

Due to high conversion efficiency, proven long term performance records, and established high volume production, crystalline silicon solar cells serve nearly 80-90% of the PV market.  In many accounts this is going to be the dominant technology for the coming years.  Therefore, reducing their carbon foot print should be a priority.  High efficiency ultra-thin wafers can meet this goal without much trouble.

References:

1.      Fthenakis, V and Alsema, E, “Photovoltaic Energy Payback Times, Greenhouse Gas Emissions and External Costs: 2004-early 2005 Status” Pog. Photovolt: Res. Appl. 14:275-280. http://www.clca.columbia.edu/papers/Photovoltaic_Energy_Payback_Times.pdf

1.      >Carbon Dioxide Emissions from the Generation of Electric Power in the United States, Department of Energy, Environmental Protection Agency, July 2000 http://www.eia.doe.gov/cneaf/electricity/page/co2_report/co2emiss.pdf